Toner carrier tubes conventionally consist of a diamagnetic material which is to be rotated about an internally situated magnetic cylinder provided with several magnetic pole pieces with axially aligned pole surfaces, situated adjacent to each other in the axial direction. The toner carrier tube, together with the magnetic cylinder, is used for the feeding of a magnetically attractable toner powder from a powder container onto a latent electrostatic image produced on a suitable support material.
In the case of magnetic cylinders it is already known to use a magnetic core that consists of several similar axially aligned adjacent permanent magnetic pole pieces with axially alilgned pole faces with a material having high magnetic permeability between at least one of the axially aligned pole faces and the toner carrier tube. In a preferred embodiment, over at least one of the axially aligned pole faces of the magnetic cylinder a strip of ferromagnetic material is attached with the width of the strip being no greater than the width of the pole face. Here it is also possible to arrange the stips of material having a high magnetic permeability not on the magnetic core but in or on a tube of diamagnetic material that is inserted between the magnetic core and the toner carrier tube and is aligned in such a way that the strips of magnetically permeable material are arranged exactly above the axially aligned pole faces on which the tube is fastened to the magnetic core or to the shaft of the magnetic cylinder.
In order to eliminate the difficulties that are associated with the construction of the magnetic poles from axially adjacent permanent-magnetic pole pieces, it has been suggested in said application Ser. No. 969,889 that the toner carrier tube have a ferromagnetic material of such a thickness or of such magnetic quality that there appears a homogenization of the magnetic field but such that the latter is only partially shunted.
This results in the advantage that no additional strips of material need be fastened to the pole faces of the magnetic cylinder. Rather it is sufficient to design the toner carrier tube so that the discontinuities of the magnetic field of the magnetic cylinder stemming from the transitions between the pole pieces be compensated for by the material of the toner carrier tube in order, in this way, to achieve a uniform and strip-free feeding of the toner powder for the development of the latent electrostatic image.